1. Field of the Invention
The present invention relates to delayed coking processes and coke drums in particular. More particularly the invention related to a system for accurately determining the froth or foam level in a coke drum on a delayed coking unit.
2. Related Information
Delayed coking processes are used in petroleum refineries to convert residuum, vacuum tower bottoms or other asphaltic process streams—normally referred to as charge on a delayed coker—into more valuable refinery products such as gasoline, diesel and gas oils. Process equipment normally associated with a delayed coker are a main fractionating tower, furnaces, coke drums, fractionation equipment, pumps, a compressor, various heat exchangers, etc. One of the products produced on a delayed coker is petroleum coke which is formed in the coke drums.
In the delayed coking processes, the residuum or other asphaltic material is circulated through a furnace for heating to coking temperature and then delivered to the coke drum where a coke product as well as the more useful products form from the thermal cracking of the heated heavy hydrocarbons. The overhead of the coke drum is quenched and circulated to a combination or fractionating tower for separation of the overhead into gas, gas oil, fuel oil, naphtha and the like. When the coke drum is filled with coke, a mechanical process of removing the coke from the coke drum is begun. Generally during the process of removing the coke from the coke drum, the circulation of the heated heavy hydrocarbon is switched to a second coke drum which fills with coke as the first coke drum is being mechanically emptied. The time between switching coke drums is the coking cycle. Most delayed coking units are operated to maximize charge rate. The variable that generally controls or dictates the charge rate is the level in the coke drum. In the vernacular of the industry that level is referred to as the foam level.
Once the foam level has reached what is considered an optimum level the charge from the furnace is switched to a second drum. The filled drum is then steam stripped for removal of residual light hydrocarbons and then cooled with water. In the process of stripping, cooling, etc., the material in the coke drum solidifies into petroleum coke which is removed mechanically.
The coke drum is designed to contain the coke and any other phenomena associated with the formation of petroleum coke. An inherent part of that process is the formation of foam in the coke drum. Downstream equipment used to recover the higher valued products from the coking process are not designed to handle the foam hence the need to assure its containment within the coke drum.
Many theories abound as to the phenomena of what is taking place in an operating delayed coker coke drum. The common theory is that the coke forms from the bottom of the drum up as the heated charge passes from the furnace into the coke drum. This theory says that “rat holes” form in the coke which allows the subsequent material to pass up through the drum.
The inventor herein has noted that the material left in the coke drum which has not been properly cooled oozes out of a coke drum after the head is removed which contradicts the theory that solid coke has formed. Also large amounts of steam, which is a vapor at the process conditions (temperature and pressure) utilized in the coking process, are injected with the charge residuum at the furnaces. If the hot residuum formed solid coke with “rat holes” as soon as it entered the drum as previously describe, an inordinate amount of back pressure for the incoming charge would develop. Further, by design 45+% of the residuum vaporizes in the coking furnaces. Ultimately as much as 70% by weight of the charge is thermally cracked to hydrocarbon vapors in the coke drum which has to flow up through the material in the coke drum into downstream recovery equipment. This vapor flow would be impeded and aggravate the back-pressure problem if solid coke was forming from the bottom of the coke drum upward as the drum is fed.